The purpose of the project is to examine the coupling between the electrical and contractile activity of the heart under voltage-clamp conditions. Sheep cardiac Purkinje fibers are voltage-clamped with the two-microelectrode technique to allow a detailed examination of how cardiac contraction depends on membrane voltage, time, and ionic currents. I intend to examine the relationship between calcium inward current and contraction, the rate and extent of recovery between contractions, and how recovery depends on preceeding electrical activity. Computer modeling and simulation are to be used to guide the live tissue experiments. A long rang goal of the modeling activity is a mathematical formulation which adequately describes cardiac muscle behavior in terms of reasonable mechanisms.